This invention relates to improvements in a light responsive circuit of a camera, in which a photo-diode is used as a light measuring element.
A light responsive circuit for a camera has been proposed, in which a photo-diode having good light responding characteristic is used as a light measuring element in place of a photo-conductive element such as cadimium sulfide (CdS). Among those attempts, there is one, as shown in FIG. 1, in which there is provided a differential amplifier A (or an operational amplifier) having a pair of input transistors T1, T2, such as field effect transistors (FET); a photo-diode PD1 is connected between the input terminals thereof; and the negative feedback is applied from the output terminal of the differential amplifier A by way of a diode D to its input terminal having the photo-diode connected thereto. The feedback brings both terminals of the photo-diode PD1 into a zero-voltage state as well as generating a shortcircuited current for the photo-diode. The amperage of the current is proportional to the brightness of light which is being received by the photo-diode PD1. The current thus produced is caused to flow through the aforesaid diode so as to obtain a voltage signal in proportion to the logarithm of the brightness of the light. For achieving a signal at a high accuracy in the prior art circuit shown in FIG. 1, it is mandatory that the characteristics of the two field effect transistors FET T1, FET T2 be in coincidence with each other. However, FET transistors having exactly the same characteristics are costly, thus leading to an increase in the cost of a camera. In addition, the circuit arrangement of FIG. 1 would be subject to a limitation in reducing a current to be consumed. This limitation is on the order of 100 .mu.A at its minimum. The circuit, therefore, fails to meet the requirement that the storage cell be capable of being used for a long period of time.
Another prior art attempt is shown in FIG. 2, in which there is shown a circuit with a relatively simple arrangement, including a single FET T3. This arrangement, however, suffers from a disadvantage in that, at the time of the light measurement of an object placed in a dark region, a higher bias voltage having an opposite direction is impressed on a photo-diode, thereby producing a high dark current. As a result, a dark current component relative to a photo-electric component will be increased in the case of a dark object, so that a signal of a high accuracy may not be achieved, with the resulting circuit having poor responding characteristics.
In operation of the circuit shown in FIG. 2, at the time of light measurement, switches S1 and S2 are closed, switch S3 is opened, and current Ip of an amperage proportional to the brightness of an incident light flows from the source of FET T3 to photo-diode PD2 towards switch S2. There appears a gate bias voltage required for flowing a photo-electric current Ip from element PD2, across both terminals of PD2, i.e., across the gate sources of FET T3. In this case, the gate has a negative voltage relative to the source. When the aforesaid gate bias voltage is charged in condensor C1 and the light measurement is interrupted, then switches S1 AND S2 are opened, while switch S3 is closed, so that a current equal to the photo-electric current Ip is continued to be fed to a load Z. In this prior art circuit arrangement, as shown in FIG. 2, when incident light on photo-diode PD2 becomes dark and accordingly a pboto-electric current is reduced, then this corresponds to the case where the gate of FET T3 is biased to a large extent, so that a higher bias voltage having an opposite direction will be impressed on photo-diode PD2. As a result, the darker the incident light, the further the dark current will increase within photo-diode PD2, thereby imparing the accuracy of a signal in a dark range, partly because of the reduction in the photo-electric current. A dark current as used herein is a current produced by a voltage having an opposite direction, rather than by the light. In addition, there is incurred a time lag due to the discharge of C1, upon abrupt change in the incident light, so that poor responding characteristics will result. On the other hand, because of the facts that a drain current is a photo-electric current Ip (including a dark current), and that the photo-electric current is small in amperage, FET T3 has to attend upon the control of a current of an extremely low amperage, resulting in fluctuations in the measurements in the dark region. For instance, a photo-electric current of several pAs is obtained for the brightness of 1/1000 lux measured on the light receiving surface of a photo-diode. It is quite difficult to consistently control such a minute current by means of an FET. The present invention is directed to avoid the aforesaid shortcomings confronted with the prior art light measuring circuit.